The mechanism of coding on magnetic tape is by a formatter device which codes and writes bits represented by magnetic flux reversals on a ferromagnetic tape medium. There are many different types of coding used in the prior art, varying according to polarities (return to zero or not during a transition), bit train compression, and clocking capability. The most common coding schemes for high-performance tapes are non-return-to-zero-inverted (NRZI), phase encoding (PE), and group coded recording (GCR) which is a combination of NRZI and PE. A code is self-clocking if a signal pulse is generated for every stored bit.
Characters are recorded on tape by tracks with each character stored in a column across the tape with embedded parity bits for error checking. Typically, each track has one write head and usually one read head. To further limit errors, information written on tape is often read immediately after being written (so-called read-after-write or RAW) by a separate read head mounted closely to the write head. On a typical tape there is a stored addressing information (SAI) section for locating a record and a data section which may also provide additional addressing information. The SAI typically includes (in sequence) a postamble immediately adjacent the previous data record, an interrecord gap (IRG) providing a space interval for tape motion changes, beginning and end of tape characters, various other markers, clocking and deskewing information, and a preamble immediately adjacent the next data record. The preamble utilizes sync marks to synchronize detection circuits for distinguishing bits. The postamble signals the end of a data record or block. To save space and access time, IRGs may be placed between blocks (IBGs) rather than records and related blocks may be grouped into a file and designated by an end of file marker. "Load point" and an "end of reel" markers indicate the beginning and end of the tape respectively and are typically reflective for detection by a photocell in the tape drive unit.
A standard format for digital data storage (DDS) using 3.81 mm digital audio tape (DAT) magnetic tape is set forth by the European Computer Manufacturers Association in the document "Flexible Magnetic Media for Digital Data Interchange" (ISO/IEC JTC 1/SC 11N 1026, hereinafter "DDS standard", 1990-07-13).
Briefly, DDS format data has two types of separator marks indicating logical separations of the data. Separator 1 is a "file mark" and separator 2 is a "set mark". User data, separator marks, and associated information are formed into groups occupying groups of tracks in a "main zone" of the track. Additional information about the contents of the group, the location of the tracks and the contents of the tracks is recorded in two parts of each track called "sub zones". The two sub zones constitute the "subdata" area of the track. In addition, there are margin zones at the extreme ends of the tape and Automatic Track Finding (ATF) zones between the sub zones and the main zone. Each zone in a track is further segmented into blocks called margin blocks (in the margin zone), preamble, subdata, and postamble blocks (in the sub zones), spacer and ATF blocks (in the ATF zone), and preamble and main data blocks (in the main zone). A "frame" is a pair of adjacent tracks with azimuths of opposite polarity (where the azimuth is the angle between the means flux transition line with a line normal to the centerline of the track). Data to be recorded is grouped into "basic groups" of 126632 bytes. Each basic group is identified by a running number from 1 to 126632. Data and separator marks are grouped into the basic groups starting with basic group no. 1. Error Correction Codes (ECC) are termed C1, C2, and C3 which are computed bytes determined from the data using Read-Solomon error correction techniques (see below for details). C3 is capable of correcting any two tracks which are bad.
Write data channel functions, including coding and error correction code, are typically performed by a controller operating through a write amplifier positioned near the write head. The write amplifier drives the write current through the write head.
Read data channel functions, including amplification and equalization of the read signals and data retrieval, are typically performed by automatic track-oriented gain-adjustment by a read amplifier and timing, deskewing, decoding, error detection and correction by a controller. The fundamental function of readback is to accurately convert the amplified read signal waveform into its binary equivalent. During writing, an external clock (oscillator) spaces recorded bits. An accurate readback therefore must be synchronous, and a code which inherently strobes the readback signal is desirable, such as self-clocking pulse generation in PE and GCR. One special type of GCR coding is 8-10 GCR which, briefly, is a coding scheme mapping each 8 bit group of data into a 10 bit code group. When the resulting 10 bit code groups are concatenated in any sequence, the resulting bit stream has the characteristic that there are never more than 3 0's in a row.
Prior art formatter systems for DDS/DAT typically require 96 to 192K bytes of static random access memory (SRAM). DDS type drives operate at 9.408 million code bits per second. In prior art read-after-write (RAW) systems, rewriting a bad frame after RAW-checking typically requires 2 to 5 intervening frames which wastes that amount of tape capacity per bad frame. Prior art systems were unreliable primarily because of multiple components and complex electronic interconnections. This resulted in relatively physically large formatter systems and high costs.
There is therefore a need for a smaller, less expensive formatter having fewer components, but achieving greater reliability and more efficient performance in terms of speed and capacity.